In electrophotographic imaging, images are formed and developed by electrostatic means. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on an imaging surface of a photoconductive element by first uniformly electrostatically charging the surface of the photoconductive element in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light-struck areas of the imaging surface are thus rendered relatively conductive and the electrostatic charges are selectively dissipated in these irradiated areas.
After the photoconductive element is exposed to the light and shadow image, the latent electrostatic image on this image-bearing surface is rendered visible by development with a finely divided colored marking material, known in the art as toner. The toner will be principally attracted to those areas on the image-bearing surface having a polarity of charge opposite to the polarity of charge on the toner particles.
The photoconductive elements, which are also referred to as photosensitive members, electrostatographic devices, photoconductors, photoreceptors, etc., are available in a variety of configurations and compositions. As disclosed in U.S. Pat. No. 4,106,934 the photoconductive element composition may be selected from a wide variety of photoconductive and insulating materials which may be single layer compositions or multi-active layer compositions.
Photoconductive elements generally have a charge transport layer and a charge generating layer superimposed on a supporting substrate. For example, U.S. Pat. No. 4,018,602 discloses charge generating and charge transferring polymeric films coated on a supportive substrate in two adjacent layers. A solvent in one of the layers causes a softening in the other layer which initiates the formation of a charge-transfer complex at the interface of the layers.
U.S. Pat. No. 4,535,042 shows the composition of a layered electrophotographic photosensitive member. An electron acceptor layer and an electron donor layer are superimposed upon each other to form a thin layer of charge-transfer complex at the interface between the two layers.
U.S. Pat. No. 4,264,695 discloses the formation of charge-transfer complexes between electron donor and acceptor molecules, which are present in separate layers. Additionally, one of the layers may contain a photoconductive pigment.
U.S. Pat. No. 4,106,934 discloses single layer and multi-layer photoconductive insulating elements. Sensitivity of the photoconductor is increased by charge transfer complexes which are obtained by combining electron donor and acceptor pairs in conjunction with a p-type photoconductor.
A composite photoconductive element may be produced from a dispersion of a photoconductive substance and an appropriate binder which is coated on a conductive substrate. For example, U.S. Pat. No. 4,543,314 discloses a process for preparing an electrostatographic photosensitive device in which a dispersion mixture is milled and applied to a substrate in an even layer and dried.
The composition of the photoconductive element is selected to form clear image reproductions. However, the slightest presence of impurities in the system may result in a variance in electrical properties which causes low quality image reproduction.
Variations in the electrical properties of a photoconductive element result in unacceptable variance in dark development potential (V.sub.DDP) and background potential (V.sub.BG). V.sub.DDP is defined as the potential on a photoconductive element in the dark a specified time after uniform charging. Unpredictable variations in this characteristic are highly undesirable, particularly for high volume, high speed copiers, duplicators and printers which require precise, stable, and predictable photoconductive element operating ranges. Erratic variations in V.sub.DDP can be unacceptable, or at the very least, require expensive and sophisticated control systems or trained repair persons to alter machine operating parameters such as charging potentials, toner concentration and the like to compensate for different photoconductive element V.sub.DDP. Failure to adequately compensate for V.sub.DDP differences can result in copies of poor copy quality. Moreover, such variations in V.sub.DDP prevent achievement of optimized V.sub.DDP properties.
V.sub.BG is defined as the potential in the background or light struck areas of a photoconductive element after exposure to a pattern of activating electromagnetic radiation such as light. Unpredictable variations in V.sub.BG can adversely affect copy quality, especially in complex, high volume, high speed copiers, duplicators and printers which by their very nature require photoconductive element properties to meet precise narrow operating windows. Thus, like photoconductive elements that exhibit batch to batch V.sub.DDP variations, photoconductive elements that have poor V.sub.BG characteristics are also unacceptable or require expensive and sophisticated control systems or trained repair persons to alter machine operating parameters. Inadequate compensation of V.sub.BG variations can cause copies to appear too light or too dark. In addition, such variations in V.sub.BG properties preclude optimization of V.sub.BG properties.
Control of both V.sub.DDP and V.sub.BG of photoconductive elements is important not only initially but through the entire cycling life of the photoconductive element. During the electrophotographic process, the photoconductive element is subjected to a series of charge and illumination steps which often produce changes in the electric and optical properties of the photoconductive element. These changes are called fatigue. Fatigue causes the operating characteristics to vary during the life of the photoconductive elements and is undesirable in actual commercial usage.
A common factor which produces variable V.sub.DDP and V.sub.BG in photoconductive elements is the small, uncontrollable variation in acidic or basic chemical impurities in the system. Additives to the photoconductive element's layer or layers may reduce or eliminate the effects of impurities. For example, U.S. Pat. No. 4,874,682 describes a monomeric or polymeric nonvolatile basic amine incorporated in a charge transport layer to eliminate the fatigue effect of acids. In another example, U.S. Pat. No. 4,725,518, the entire disclosure of which is incorporated by reference herein, discloses addition of an aromatic amine compound and a protonic acid or Lewis acid in a charge transport layer to control V.sub.DDP and V.sub.BG.
Another known treatment of photoconductive elements to control acidic or basic variations affecting V.sub.DDP and V.sub.BG involves doping the photoconductive element with other acids and bases. For example, a variance in V.sub.DDP and V.sub.BG may be controlled by the addition of trifluoroacetic acid to the transport layer in amounts ranging from about 0.1 to 100 ppm. However, the actual amount varies and must be determined by frequent measurement during the manufacturing process of the electrical behavior of the device. The dopant content is readjusted to compensate for the quantity of acid necessary to achieve the desired electrical specifications. This acid doping procedure is tedious, time-consuming and difficult to predictably control.
Thus, it would be desirable to improve manufacturing processes of photoreceptors to eliminate effects of acids and bases in the photoconductive element. Further, it would be desirable to eliminate the need for variable amounts of acid or base dopant to impart the desirable electrical behavior.